1. Field of the Invention
The present invention relates to non-volatile memory devices and relates more particularly to a process and a device enabling the storage of analog quantities.
2. Description of the Prior Art
Non-volatile memories with electric writing and erasure, known as EEPROM memories, which is short for "Electrically Erasable and Programmable Read-Only Memory", have an MOS transistor structure, the gate of which transistor is completely insulated (that is to say floating). The voltage state of this gate, which determines the conduction of the MOS transistor, may be modified by injecting thereinto or extracting therefrom a certain quantity of electric charges. A known technique for modifying the charge of the floating gate consists of producing a current, by a tunnel effect via an insulator of very slight thickness, between a doped zone of the substrate, known as an injection zone, and the floating gate. This tunnel effect, also called field emission or Fowler-Nordheim effect, is produced as soon as a sufficiently high voltage exists between the injection zone and the floating gate. As the latter is insulated, application of the required voltage is effected between the injection zone and a control electrode which is coupled capacitively to the floating gate. A memory of this type using the Fowler-Nordheim effect for erasure has been described in U.S. Pat. No. 4,399,523.
Most of the known applications only consider the information stored in the memory as information of the binary type, even though information of the analog type can be obtained by converting several bits thus stored. However, it has already been proposed, in the article by E. Sackinger and W. Guggenbuhl which appeared in the IEEE Journal of Solid-State Circuits, Vol. 23, No. 6, December 1988 and entitled "An Analog Trimming Circuit Based on a Floating-Gate Device", to use the analog quantity stored in an EEPROM memory cell, especially to achieve circuit adjustments. However, the programming techniques used, consisting of applying to the electrodes of the memory continuous or high pulsed high voltages of a definite sign, only allow precise fixing of the charge existing at the floating gate by having recourse to an iterative process of successive measurements and corrections, which must be implemented for each individual cell. As the floating gate is not directly accessible, implementation of this adjusting process has shown itself most often to be delicate and requires, in all cases, additional circuit elements. Moreover, the fact of having to adjust the stored charge limits further the applications of these memories in the analog domain.